1. Field of the Invention
This invention relates to a process for producing molded products of propylene resin which are light in weight and excellent in strength, and are suitably used in the fields of industrial parts, especially, automotive parts, domestic electrical appliance parts and containers, packages or the like.
2. Background Art
So far, propylene resins, especially because of their excellent mechanical strength and processibility, have been widely used in the fields of industrial parts such as automotive parts.
Especially for those industrial parts which are required to have high rigidity and heat resistance, propylene resins have been used in composite with inorganic fillers such as glass fibers. Such propylene resin compositions have been practically used for automotive parts, more specifically, interior trim parts such as trims and instrument panels, exterior trim parts such as bumpers and functional trim parts such as fan shrouds.
A problem with the conventional propylene resin compositions is that since the compositions used are in composite with large amounts of inorganic fillers such as talc or glass fibers, which are much higher in density than propylene polymers, their own densities become too large, thus providing molded products of too much weight. In order to overcome this problem, molded products made from propylene resin compositions filled with light-weight fillers having a very low true density have been proposed in the art. Among others, particular interest is given to injection-molded products made from propylene resin compositions filled with fillers in the form of hollow particles.
For instance, Japanese Patent Laid-Open Publication No. 51-82367 teaches the technique of foam molding hollow filler-containing resins by injection molding.
Japanese Patent Laid-Open Publication No. 1-294751 has proposed to use propylene polymer compositions containing fillers in the form of hollow particles having a very high pressure resistance.
However, the propylene resin compositions with such fillers in the form of hollow particles, e.g., hollow glass spheres, have the drawback that the hollow particles are likely to be broken during kneading and molding processes whereby it is difficult to attain considerable reduction in weight of mold products, especially injection-molded products.
It is therefore a primary object of the present invention to provide a process for producing molded products of propylene resin compositions containing fillers in the form of hollow particles, which effectively prevents the hollow particles from being broken during molding processes and can provide molded products light in weight and excellent in strength.